I need something that can give me a decent preview and will help me do some manual corrections.

I thought that you might need a CSS link inserted in every file, a big job, so I misunderstood what was needed.

However, I do have a suggestion for what might do the job for you. A Content Management System ("CMS") does just what you are asking for, letting the user pick up each file, preview it, and make some manual corrections in a WYSIWYG format. I am learning to use SNIPPETMASTER for just those features. Check it out, it is free or very inexpensive, depending on features.